Switching converters are widely used to convert an input signal into an output signal through the ON and OFF switching of switches. A controller needing a DC power supply voltage, such as a control IC, is generally used to control the switches in the switching converter. When the switching converter is just started up, there needs some time (startup time) to establish the power supply voltage. During the startup time, the switching converter can not work normally.
In most applications, the startup time of the switching converter needs to be minimized. For TRIAC dimming LED drivers, fast startup is especially needed. Since the on-time of the TRIAC is very short in the minimum dimming angle, the startup time of the switching converter will directly limit the dimming depth.
FIG. 1 illustrates a prior switching converter. A rectifier bridge is used to convert an AC input voltage Vin into an uncontrolled DC voltage. A flyback converter comprising a transformer T1, a switch M, a diode Dout and a capacitor Cout converts the uncontrolled DC voltage into an output signal to drive a load. A controller 101 is used to control the ON and OFF switching of the switch M. One terminal of a resistor R1 is coupled to the output terminal of the rectifier bridge. A capacitor C1 is coupled between another terminal of the resistor R1 and the ground. The voltage across the capacitor C1 is used as the power supply voltage of the controller 101. When the switching converter is just started up, the capacitor C1 is charged by the input voltage Vin through the rectifier bridge and the resistor R1. When the voltage across the capacitor C1 is increased to be sufficient to start up the controller 101, the switching converter enters into normal operation. The auxiliary winding of the transformer T1 provides power to the controller 101 through a diode D1. To minimize the startup time, one possible solution is to reduce the resistor R1 or the capacitor C1. But a smaller R1 will increase the power loss, and lead to lower efficiency and higher temperature on this resistor. The capacitor C1 also can not be reduced a lot since it needs to hold the power supply voltage.
FIG. 2 illustrates another prior switching converter. The controller 201 comprises a high voltage current source l1 and a switch S. The capacitor C1 is charged by the high voltage current source l1 through the switch S when the switching converter is just started up. After the switching converter enters into normal operation, the switch S is turned off. The high voltage current source l1 is disconnected from the capacitor C1, and the auxiliary winding provides power to the controller 201 instead. By applying the method shown in FIG. 2, fast startup and high efficiency can be got. However, the switch S is a high voltage device, which is high cost and also hard to be integrated.